1. Field of the Invention
The present invention relates to a method for determining the position and/or the movement of at least one object in the surroundings of a means of transportation, in particular of a vehicle, and/or for transmitting information with the aid of at least one transmitted acoustic signal, and a surroundings detection device for carrying out the method according to the present invention.
2. Description of the Related Art
Ultrasonic systems which measure in pulses are presently conventionally used for the acoustic surroundings detection of vehicles. Due to a high level of mechanical robustness, their operating range is generally restricted to a small frequency range. They have a high quality. It is known to those skilled in the art that systems having a high quality require a relatively long time until they have dissipated an oscillation amplitude again after an excitation. The time which is necessary to dissipate the excitations is referred to as decay. Electroacoustic transducers of sensors, which are used both for the emission and for the subsequent reception, require a preferably short decay to also allow a measurement at close range with the aid of pulse runtime measurement.
The decay begins with the end of the transmission pulse generation and ends when the oscillation amplitude is significantly less than the amplitude of potential echoes and/or signals to be received.
The decay may be changed, on the one hand, by external influences, as are shown in FIG. 1, or, on the other hand, by internal-sensor features such as damage to the electroacoustic transducer, for example, by road stone.
FIG. 1a shows a vehicle 10, which has at least one sensor 20, which is covered by a thin ice coating 30. FIG. 1b shows a vehicle 10, whose rear sensors 40 are covered with a layer of powdered snow. Ice coating 30 in FIG. 1c extends over only a part of the diaphragm of the transducer of sensor 20. FIG. 1d again shows a vehicle 10, which has a sensor 20 at the right rear, which is shielded by a snow curtain 40. FIG. 1e shows a side view of vehicle 10 from FIG. 1d, sensor 20, which is installed in bumper 25 and is shielded by snow curtain 40 at a distance of approximately 20 mm in front of the bumper, again being visible.
In the case of the described sensors, it is preferred that they are subjected to a manufacturing test and that they have an independent monitoring function for studying the functional capability of the measuring system formed by the sensors during operation. With the aid of the independent monitoring function, the sensors may be put into a safe operating state, in particular in the event of a decrease of the sensor performance or in the event of damage, which may occur as a result of external disturbances, for example, such as damping due to a coating.
It is not only important to achieve an improvement of the sensor performance in the range of the measuring capability for distances beyond 20 cm, but rather also to be able to detect in the range of the object shape differentiation and the close-range measuring capability at distances of 5 cm or less, for example, to also be able to detect a snow curtain or a coating on the sensor, which cover sensors, as shown in FIGS. 1d and 1e. 
These aspects were heretofore only checked on the basis of a measurement of the decay duration.
Alternatively, these aspects could also be checked with the aid of a frequency measurement. For example, the attempt has been made with the aid of a frequency measurement to determine the errors in the decay which occurred during the sensor manufacturing with the aid of bandwidth estimation. For this purpose, a system (surroundings detection device) 50 according to FIG. 2 is operated, in which a frequency measurement is carried out with the aid of an analysis unit 55. FIG. 2 schematically shows a surroundings detection device 50, having an acoustic unit 60 for transmitting and/or receiving acoustic signals 70 of today's ultrasonic sensors 75.
A transmission pulse generator 80 generates a pulsed transmission signal (not shown), in that signal generator 80, during the transmission pulse duration, outputs an electrical signal having signal frequency f to electroacoustic transducer (acoustic unit) 60. This transducer 60 begins to oscillate at frequency f of the transmission pulse and therefore to output an acoustic transmission pulse 70. Depending on the quality and resonant frequency fR of transducer 60 and the frequency locations of the frequency-determining components surrounding transducer 60, in the case of this type of frequency measurement, objects at different distances are often recognized by sensor 75, in particular upon variation of the sensitivity threshold. The experimental concept was based on the assumption that sufficiently good conclusions about sensor features could be drawn solely from the oscillation amplitude of the echoes at various signal frequencies with the aid of this type of frequency measurement.
Undisturbed sinusoidal signals may be described by equation (1) represented as follows:r(τ)=A1·sin(2πf·τ+φ)+A0  (1)
In the frequency range, the features of periodic sinusoidal signals are determined with the aid of filters, while direct component A0 is obtained by averaging. The most important features of filters for periodic signals are filter frequency fh, bandwidth B, and the phase dependence. The closer frequency f of the periodic signal to be studied is to filter frequency fh, the more strongly such a filter oscillates in comparison to the filters of other filter frequencies fh.
Bandwidth B=fO−fU describes the frequency range fU≦fh<fO around filter frequency fh in which signal frequency f may vary, without the strength of the oscillation, (i.e., the deflection amplitude of the filter) having significantly decreased, in particular so that the strength of the oscillation decreases by not more than 3 dB. At a given signal amplitude A1, the strength of the oscillation increases with the decrease of bandwidth B. This feature is also summarized by the parameter designated as quality Q=fh/B.
Filters which are independent of the phase relation of the signal to be studied to the phase relation of the observing measuring system and therefore do not have a phase dependence are referred to as incoherent filters. In contrast, phase-dependent filters are referred to as coherent filters. If the phase of the signal to be studied rotates in relation to that of the measuring system in the case of coherent filters by 90°, the oscillation state of these filters thus changes from maximum deflection in resonance to rest or vice versa.
The less the bandwidth of the filter, the longer it takes, however, until the filter has settled. This phenomenon may also be referred to as fuzziness of the frequency detection. To be able to reliably detect a frequency with the aid of a filter, the time until reaching the settled state of the filter is to be waited out, which generally lasts more than one period.
The time required for determining parameters φ, f, and A1 may be decreased if the gradient of the oscillation intensities is analyzed with the aid of a bank of filters having filter frequencies fh slightly offset to one another.
All frequency measurements have the disadvantage that they require multiple periods until changes in the signal are recognized. In particular, frequency measurements which require an excitation using signal shapes of various carrier frequencies are disadvantageous, for example, as in the case of the above-explained bandwidth estimation.
An important requirement may be recognized from FIG. 1e, specifically the close-range measuring capability of an ultrasonic sensor. Nearby objects, or even curtains in front of the transducer, which do not massively influence the resonance properties of the transducer, may not be recognized with the aid of a frequency measurement, since the echoes already come back to the transducer during the decay of the transducer, on the one hand, and the signal at the transducer is only so slightly varied by such objects, on the other hand, that these changes may not be exposed sufficiently solely with the aid of a form of the runtime-dependent amplification, as is known from the publication published international patent application document WO 2010/076061 A1.
A coating, in particular made of ice or mud, which is 1 mm in front of the transducer, for example, also may not always be sufficiently recognized with the aid of a frequency measurement by way of a pulse duration modulation, which is known, for example, from the publication published European patent application document EP 2 251 710 A2. A system and a method are described in the publication EP 2 251 710 A2, which relate to emitting ultrasonic pulses of different pulse duration (pulse duration modulation) and pulse strength, to monitor partial areas located at different distances from the sensor system.
An intrapulse analysis is known from the publication published international patent application document WO 2011/009786 A1, which is used for surroundings detection. Acoustic alternating signals are used, which may therefore be described as a period duration sequence having a time-variable signal strength. The intrapulse analysis includes the determination of a measurement sequence of period durations of a received acoustic signal, which, similarly to a fingerprint, is a characteristic both for the emitted transmission pulse and for the transmission path to a receiving unit, respectively. If an analysis unit knows the fingerprint which is characteristic for a transmission scenario, which is preferably represented by a sequence of reference period durations, it may thus be detected on the receiver side which scenario was present in the case of a received acoustic signal.